The present invention relates to a switching device for the interruption of a fault current, preferably for a transformer branch in a medium voltage switching system; and more particulary to such a switching device which is suitable for the performance of repeated interruptions and is actuated by an energy store including at least one turn-off spring and an actuating device which becomes active upon the occurrence of a fault current.
Such a switching device is employed with preference in closed line switching systems which are generally composed of two closed line loops and at least one switching panel for a transformer branch. A known switching device of the foregoing type is disclosed in a publication by CALOR-EMAG, entitled "Schaltanlage fur Ortsnetzverteilerstationen mit Vakuum-Leistungsschalter" [Switching System For Local Network Distributor Stations Equipped With A Vacuum Power Switch], by Klaus Bottger and Bruno Schemann, Special Printing from Elektrotechnische Zeitschrift {Electrical Engineering Magazine], Volume 106, No. 10, (1985). The closed line terminals are each equipped with a power circuit breaker and the transformer tap with a vacuum power switch. The latter, in conjunction with an excess current relay fed by a current transformer, provides short-circuit protection for this branch.
The cited switching system, which is insulated by SF.sub.6 gas, is considered to be fully insulated since the short-circuit protection is no longer provided by EHV (extra high voltage) fuses disposed outside the encapsulation. Moreover, the switching system is considered to be maintenance free since, after the interruption of a short-circuit current, a possibly remote, controllable renewed turn-on of the power switching device permits resumption of operation without having to exchange an operating medium, such as, for example, the fuses. The absence of EHV fuses additionally precludes further sources of error right from the start, such as the existence of a critical current range in which a fuse fails to respond, or responds only after a very long period of melting, and in which therefore the danger of overheating and explosion exists. Moreover, it is noted, among others, that safety characteristics fluctuate considerably and that the power circuit breaker is overloaded in the borderline region of the fuse.
On the other hand, in the cited switching system, the vacuum power switch must have the full insulating capability as specified, for example, by applicable VDE [Vorschriftenwerk Deutscher Elektrotechniker=Rules for German Electrical Engineers] rules and must be able, by the layout of its contacts and the energy store, to turn off short-circuit currents without welding.
In the connection region, that is, outside of the encapsulation of the switching system, sufficient space must be provided for the necessary cable transformers. The excess current relays must additionally be checked a certain time intervals in routine monitoring which is possible only if the branch can be separated. With the prior art switching device, a three-phase interruption of the branch is made even if the fault is only in a single phase, although in many cases it would be of advantage to maintain two-phase emergency operation. This applies primarily for rigidly grounded networks as they are used in many countries overseas. If one employs the prior art switching device in a switching system that is insulated by air, all its elements are fully exposed to the climate of the environment.